1. Field of the Invention
The present invention relates to a capacitive electromechanical transducer that transmits and/or receives elastic waves, such as ultrasonic waves.
2. Description of the Related Art
A capacitive micromachined ultrasonic transducer (CMUT), which is a capacitive electromechanical transducer, is proposed as a transducer that transmits and/or receives ultrasonic waves (refer to PCT Japanese Translation Patent Publication No. 2003-527947). The CMUT can be produced through a micro-electromechanical system (MEMS) process to which a semiconductor process is applied. FIGS. 3A to 3C are schematic views of a MEMS; FIG. 3A is a top view; FIG. 3B is a sectional view taken along line IIIB; and FIG. 3C is a sectional view taken along line IIIC. FIGS. 3A to 3C illustrate a vibrating membrane 101, first electrodes (upper electrodes) 102, supporting parts 105, gaps 106, second electrodes (lower electrodes) 107, and a substrate 108. In the CMUT, first electrodes 102 are formed on the vibrating membrane 101. The vibrating membrane 101 is supported by supporting parts 105 formed on the substrate 108. On the substrate 108, the first electrodes 102 are formed on the vibrating membrane 101, and the second electrodes 107 opposes the upper electrodes 102 with the gaps 106 (which are each usually 10 to 900 nm) provided therebetween. In FIG. 3, the vibrating membrane 101 sags toward the substrate 108 due to an external force. Each pair of electrodes opposing each other with the vibrating membrane 101 and one of the gaps 106 interposed therebetween is referred to as a cell. The CMUT, which is a transducer array, includes around 200 to 4000 elements, which each include a plurality of cells (usually around 100 to 3000 cells). The actual size of the CMUT is typically around 10 mm to 10 cm.
In the CMUT, all of the first electrodes 102 are electrically connected. The vibrating membrane 101 has areas P (represented by the hatched areas in FIG. 3A) in which the first electrodes 102 are not formed. The vibrating membrane 101 has such areas P to decrease its electrode area, which particularly influences the vibration characteristic, to a size that does not significantly affect the transmission and/or reception efficiency. The thickness of the first electrodes 102 formed on the vibrating membrane 101 is approximately one submicron, which is not ignorable with respect to the vibrating membrane 101 having a thickness of approximately 0.1 to 1.0 μm. Consequently, the first electrodes 102 have a significant effect on the vibration characteristic of the CMUT. Thus, the thickness of the first electrodes 102 on the vibrating membrane 101 is to be minimized. However, when thin first electrodes 102 are provided, the wiring resistance component of the electrodes becomes large, causing a nonuniform distribution of the electrical potential applied to the first electrodes 102 on the surface of the CMUT. During transmission and/or reception operation by the CMUT, a predetermined electrical potential is applied to the first electrodes 102, causing a difference in the electrical potentials of the first electrodes 102 and the second electrodes 107. This electrical potential difference generates an electrostatic attractive force, which is the external force, between the first electrodes 102 and the second electrodes 107, causing the vibrating membrane 101 to sag toward the substrate 108. Transmission and/or reception of ultrasonic waves are performed in this state. The amount of sagging determines the transmission and/or reception efficiency of ultrasonic waves. Therefore, when a nonuniform electrical potential distribution is generated on the surfaces of the first electrodes 102 of the CMUT, the amount of sagging of the vibrating membrane 101 changes, causing a fluctuation in the transmission and/or reception characteristics of the CMUT. This fluctuation causes degradation in the quality of images reproduced on the basis of information of the ultrasonic waves.